Geological formations form reservoirs for the accumulation of hydrocarbons in the subsurface of the earth. Such formations contain networks of interconnected paths in which fluids are disposed that ingress or egress from the reservoir. Knowledge of both the porosity and permeability of the geological formations are useful to determine the behavior of the fluids in this network. From information about porosity and permeability, efficient development and management of hydrocarbon reservoirs may be achieved. Considering that hydrocarbons are electrically insulating and most water contains highly conductive salts, resistivity measurements are a valuable tool in predicting the presence of a hydrocarbon reservoir in the formations.
One technique to measure formation resistivity involves the use of electromagnetic induction using transmitters of low frequency magnetic fields that induce electrical currents in the formation. The induced currents in turn produce secondary magnetic fields that are measured in an adjacent wellbore (or at some distance away in the same wellbore) by a magnetic field receiver.
The performance of a magnetic field receiver or a magnetic field transmitter positioned within a wellbore casing may be compromised by the casing's effect on the magnetic field to be measured. Specifically, the measurable magnetic field induces a current that flows concentrically about the receiver coil and tends to reduce the magnetic field within the casing. The magnetic permeability of the casing also acts to distort the magnetic field and influences the behavior of the currents. The measurable magnetic field may be highly attenuated as a result and the measurements made by the receiver may be influenced by variations in attenuation caused by variations in the casing's conductivity, permeability, thickness and diameter. Often, a cased wellbore reduces the magnetic field signal to a level that is undetectable by standard receivers. Moreover, the variance in conductivity, permeability, thickness and diameter along a longitudinal axis of a length of casing makes it difficult to determine an attenuation factor (which represents attenuation of the measurable magnetic field caused by the casing) at any selected point. The inability to determine an attenuation factor at a selected point along the casing may cause errors in field measurements that are not easily corrected.